The invention relates to a reactor for direct utilization of external radiation heat for thermal or thermo-chemical material processes.
In many chemical processes, solids need to be heated to high temperatures for the chemical reaction to proceed. If the reaction temperature is exceeding the operating temperature of conventional metals like steel, the apparatus has to be made from alternative high-temperature resistant materials. Examples of such energy-intensive high-temperature processes with considerable CO2 emissions are the processing of lime and cement as well as the reduction of metal oxides.
Different types of solar reactors are known, even solar rotary kilns. However, they usually utilize direct sunlight as heat source. The solar energy is concentrated in a solar concentrator and focused through an aperture into the reaction chamber that is often tightly closed by a transparent window. Such applications locally yield high energy densities, Therefore, it is the aim of the present invention to provide a reactor of the mentioned type that can be heated by an external radiation heat source, in particular concentrated solar radiation, which is particularly well suited for the efficient utilization of direct radiation heat transfer for thermal or thermo-chemical processing of different materials at high temperatures.
This aim is achieve according to the present invention by a reactor for direct utilization of external radiation heat for thermal or thermo-chemical material processes, comprising:                a) a containment can having a longitudinal axis,        b) an external driving mechanism for controllable rotating the containment can,        c) a cavity having a substantially cylindrical shape, said cavity being at least partially lined with an insulation layer and being disposed in said containment can,        d) said cavity having at least one gas inlet and at least one gas outlet and an aperture for allowing to insert the external radiation heat into the cavity,        e) a feeder mechanism being moveable along the longitudinal axis into and out of the cavity for supplying the material subject to the thermal or thermo-chemical material process;        f) said at least one gas outlet is formed as a ring channel tube having an inner and an outer surface; whereby the feeder mechanism is at least partially hosted within a tube cavity defined by said inner surface; and        g) cooling means being associated with said inner and/or outer surface.        
These features lead to a reactor that delivers the heat directly into the cavity for the thermal or thermochemical processing of the feed material that is homogeneously heated by controlling the rotation speed of the containment can position. In contrast to conventional processes, a homogeneous and clean thermal or thermochemical processing of the feed materials is ensured. By defining a heat gradient along the annular shaped gas outlet by the cooling means allow to collect the desired reaction products outside of the cavity.
The mechanical feeder system is crucial for the supply and the distribution of the reactant particles throughout the cavity.
In order to provide a broad range of flexibility said feeder mechanism can be formed as a screw conveyor. This mechanism enables the operator of the reactor to match current solar conditions and the chemical reactivity of the reactants so that the reactor can operate under optimal conditions. The feeding can thus be one ranging from continuous to batch operation.
In order to prevent a relevant heat transfer into said feeder mechanism, a thermally insulated head section can be provided which shields the water cooled screw conveyor from the hot cavity when there is no feeding. This arrangement helps reduce heat loss from the cavity to the water cooled feeder.
For the controlled gathering of gaseous reaction products the inner and/or outer surface of the ring channel gas outlet can be cooled by the cooling means to generate a negative temperature gradient as seen in the flow direction of a reaction gas flowing from the cavity through the gas outlet. To support this gathering at distinct locations or within a distinct range of the gas outlet ring channel, cooling inert gas can be introduced into the ring channel, such as an inert gas like Ar and/or nitrogen. This measure allows the operator of the reactor to design the temperature gradient so that at least one reaction product from the chemical or thermo-chemical material process is condensing and/or solidifying on the cooled inner and/or outer surface within the ring channel.
A suitable measure for the removal of the condensed or solidified reaction product is to abrade it from the inner and/or outer surface by scraper means that can optionally and in a preferred embodiment be disposed on the inner and/or outer surface being activated by the movement of the feeder mechanism. The feeder can also be retracted toward the back of the cavity. When this occurs the inner and/or outer surface of the feeder is swept by a scraper and the particles that formed on the feeder's cold surface are dragged out of the reactor where the gas also exits the reactor.
To increase the lifetime of the insulation layer defining the cavity, the insulation layer may comprise a protective barrier towards the cavity in order to prevent the reaction material from damaging the thermal insulation around the cavity. The barrier is primarily for preventing the gaseous products from being lost in the insulation. In a preferred embodiment, the protective barrier is a thin layer selected from the group consisting of silicon carbide, or hafnium oxide and possibly thorium oxide or other suitable materials.
In order to maintain an efficient chemical or thermo-chemical process an inert gas can be introduced at the gas inlet into the cavity, said gas being preheated to prevent reaction products from condensing in the hot cavity.
The quality of the feed material and the quality and/or quantity demands for the reaction product may differ from case to case what requires a certain flexibility with respect to the process parameters. Therefore, the containment can be rotated at a rotational speed in the range of 20 to 200 rpm, preferably in the range of 80 to 150 rpm.
In order to simplify the constructive measure of the reactor said containment may preferably rotate along its longitudinal axis. It should be mentioned that the containment can may alternatively rotate along an arbitrary axis.
For the homogeneity of the process and the flow of the feed material a reaction product originating from the feed material is continuously removed through the gas outlet. Therefore, the ring channel is shaped as a collector for collecting the reaction product even when the containment can is rotating.
By collecting the removed process gas and preheating the feed material with the collected process gas the energy balance of the process can be improved.
The lifetime of the reactor and the homogeneity of the temperature distribution both are significantly a function of the material properties of the used materials. made from a high-temperature resistant and thermally nonconductive ceramic material.
The external radiation entering the cavity assembly through the aperture may have an average energy flux in the range of 150-500 Watts/cm2, when concentrated solar radiation is used. This power density has to be transferred into the cavity with a certain focussing but even with a certain homogeneous distribution. Therefore, a heat shield is provided to protect the edges of said aperture from said concentrated external radiation heat. The heat shield can be made from a heat resistant material or can be made from a fluid cooled metal, preferably aluminium or copper. Additionally, the heat shield can be suitably ring-shaped and comprises said aperture of said cavity. To diminish the leakage of heat from the cavity, said cavity may comprise a gas-cooled transparent window for said incident external radiation heat. For the purpose of increasing the solar gain, said aperture may comprise a secondary concentrator.
The reactor is generally suited for all thermal or thermochemical material processes which allows to use as the feed material any type of organic, inorganic, metallic, or ceramic solids, as well as mixtures of them, which are then subject to said thermal or said thermochemical material processing. Especially; fine-grained materials, such as zinc oxide, are preferred to generate a reaction product, such as zinc. Therefore, the reactor according to this invention is particularly suited for the clean processing of a variety of fine-grained materials at temperatures exceeding 1500 K. Utilizing this invention, some processing operations, which are currently carried out in direct-fired rotary kilns, for example that of lime production, may be more economical due to energy savings and other benefits of the reactor design of this invention.
Further advantageous features result from the dependent claims, the following description, and FIG. 3 described below.